1. Field of the Invention
The present invention relates to a process for chlorinating ethylene carbonate to produce tetrachloroethylene carbonate. The latter is decomposed to produce oxalyl chloride and phosgene.
2. Background Information
Oxalyl chloride (COCl).sub.2 is a useful reagent, for example as a chlorinating agent, in a large number of processes in the chemical and agricultural industries. Heretofore, oxalyl chloride has most often been produced by a solid phase reaction between phosphorous pentachloride and oxalic acid according to the equation: EQU (COOH).sub.2 +2PCl.sub.5 .fwdarw.(COCl).sub.2 +2POCl.sub.3 +2HCl.
This reaction is difficult to control, as it is a solid-solid reaction and involves both endothermic and exothermic stages. The yields of oxalyl chloride are unpredictable and usually very low, in the order of 30-50%. Further, greater than stoichometric quantities of phosphorous oxychloride are produced. This by-product is hazardous and, unless produced in very large amounts, represents a difficult waste product to dispose of or store.
Another process for producing oxalyl chloride is disclosed in U.S. Pat. No. 2,816,287 issued Dec. 10, 1957, and assigned to E. I. duPont de Nemours and Company. The process involves photochemically chlorinating ethylene carbonate (I) to form the tetrachloro derivative (II) and subsequently decomposing the tetrachloroethylene carbonate (II) to oxalyl chloride (III) and phosgene gas (IV) as represented by the following equations: ##STR1##
Under the conditions set forth in the duPont patent, the above process, in the inventors' experience, gives unpredictable by-products and low yields in both the chlorination and decomposition stages. Also, in scaling up the process from the bench top examples given in the duPont patent, to a commercial scale, the inventors needed to overcome many unpredictable problems.
More particularly, the duPont patent teaches chlorinating ethylene carbonate by bubbling chlorine into a refluxing mixture of ethylene carbonate and carbon tetrachloride in an illuminated flask equipped with a stirrer. The carbon tetrachloride is included in a very large amount, and thus controls the reflux temperature of the above reaction to about 75.degree.-85.degree. C. The reaction is judged to be complete when the yellow color of free chlorine persists for 30 minutes after cessation of chlorine addition. The tetrachloroethylene carbonate product is fractionated from the carbon tetrachloride solvent.
A number of problems are associated with the use of such a large excess of the carbon tetrachloride solvent. The solvent cuts down on the productivity of a given reaction vessel, reduces the light penetration, increases the reaction time and increases the overall costs of the process. Further, the tetrachloroethylene carbonate product must be fractionated from the solvent at the end of the reaction before proceeding to the decomposition step.
Another problem with the above process concerns the use of mechanical stirrers. The corrosive nature of the chemicals involved is sufficient to corrode metal and even teflon coated stirring devices.
Further, in scaling up this photochemical chlorination process to a commercial scale, it is difficult to achieve good light penetration. As the size of a flask-type reaction vessel is increased, the ratio of surface area to volume decreases. Thus light transmission is reduced, and the reaction time is increased. The duPont reference gives no information concerning a large scale operation of their process.
In French Pat. No. 1,363,740, issued May 4, 1964, and assigned to Chemische Werke Huls Aktiengesellschaft, a process is disclosed for preparing tetrachloroethylene carbonate wherein ethylene carbonate is photochemically chlorinated in the absence of a solvent. While the above patented process overcomes the problems associated with the use of a large excess of solvent, it does not confront the problems of stirring and illuminating the reaction mixture on a large scale.